THE BUDA LIMESTONE OF BIG BEND NATIONAL PARK: AN INTEGRATED STUDY OF THE BIO-ISOTOPE AND SEQUENCE STRATIGRAPHY OF AN INDURATED CARBONATE

Within Big Bend National Park (BBNP), the biomicritic Buda Limestone consists of a lower marl member and an upper massive wackestone member. The contact between the Buda and overlying Boquillas Fm. has been interpreted as a subaerial exposure based on a microkarst surface (Lock and Peschier, 2006; Lock et al., 2007). It is possible that the Buda is bounded by subaerial unconformities, and that it represents its own depositional sequence; however, Lower Cenomanian and the Upper Cenomanian ammonites have been reported from the lowermost Boquillas, suggesting that the Lower Cenomanian fauna may be reworked (Young 1958; Cooper et al., 2008). This study combines thin-section foraminiferal biostratigraphy with stratigraphic surface and paleoenvironmental interpretation to investigate the Buda – Boquillas contact. If the upper Buda was subject to subaerial processes, a negative δ¹³C shift would be expected at the contact, along with a positive δ¹³C shift in the overlying unit (Allen and Mattews, 1982; Railsback et al., 2003; Suarez et al., 2009). The requisite δ¹³C profile was not observed in the upper Buda and lower Boquillas, and little petrological evidence for significant dissolution at the Buda – Boquillas contact was observed. The ammonite zonation for the basal Boquillas is in agreement with the only foraminiferal zonation of the Boquillas made in BBNP by Frush and Eicher (1975). Using washed samples, their study places the basal Boquillas in the Middle Cenomanian R. cushmani-Th. greenhornensis Subzone. A thin-section analysis of the indurated carbonate beds in the basal Boquillas has revealed the double keeled Dicarinella sp. Double keels did not evolve until the R. cushmani-D. algeriana Subzone, thus constraining the Boquillas to the Late Cenomanian. The Buda Limestone contains a shallow pelagic-dominated fauna with 45-90% heterohelicids, and fewer numbers of spiral forms. This study places the Buda within the Lower to Middle Cenomanian Th. globotruncanoides Zone, and is in agreement with ammonite data (Cobban et al., 2008). This study evaluates the possibility of a subaqueous unconformity, with older fauna being reworked into the basal Boquillas. This model allows the Buda and Boquillas to be related in terms of sequence stratigraphy, since eustatic sea level need not have produced the unconformity.